A cellular mobile telecommunication system includes a mobile station communicating with any one of a plurality of geographically spaced base stations in a wireless network. The wireless network provides communication on both analog control and voice channels and digital control and voice channels. Depending upon the type of service a user subscribes to not all of these channels are used. Older cellular systems included only analog control and voice channels (ACC and AVC, respectively). Subsequently, digital voice channels (DVCH) were added. However, only newer digital phones could communication on the digital voice channels. More recently, wireless network systems have added digital control channels (DCCH) which provide additional control features. Again, only a phone configured to communicate on a digital control channel is capable of doing so. Nevertheless, all of the phones are downwardly compatible so that even digital-type phones can communicate on analog voice and control channels.
Due to network system design, an analog control channel tends to be easier to acquire than a digital control channel. Even mobile stations equipped to communicate with digital control channels will often initially acquire service on an analog control channel. However, digital control channels are the preferred service provider for such mobile stations, referred to as IS-136 mobile stations. Digital control channels are preferred because they provide more features and longer standby time, among other factors. An IS-136 TDMA cellular system provides the capability for an ACC to contain a control channel information message in its overhead message train (OMT) that serves as a pointer to a DCCH for IS-136 compatible mobile stations. These mobile stations can use the DCCH pointer information to facilitate a search for digital service, since ACCs are standardly confined to a subset of frequencies defined in IS-136, whereas DCCHs can be located at any IS-136 defined frequency.
A mobile station that can use the DCCH pointer normally acquires synchronization on the DCCH and then proceeds to read the overhead information to determine whether or not to remain on the DCCH. A DCCH has to be suitable from both signal strength and service perspectives. The broadcast information provides a minimum received signal strength (RSS) necessary for the mobile station to camp on the DCCH. If the mobile station's RSS for the given DCCH is not high enough to meet the broadcast criteria, then the mobile station leaves the channel to attempt to acquire service on another channel. This channel will typically be the analog control channel from which the mobile station obtained the DCCH pointer. Once the mobile station is back on the ACC, it will continue to receive the DCCH pointer with a maximum periodicity of five seconds.
If the mobile station could not camp on the DCCH solely because of received signal strength, then it is desirable for the mobile station to recheck the camping suitability of the DCCH sometime in the future since signal conditions tend to be very dynamic. Prior solutions have rechecked the camping suitability of the DCCH each time an OMT is received on the ACC. With this method, the mobile station would only remain on an ACC for a maximum of five seconds. The mobile station might not receive pages for incoming calls and have difficulty originating a call. Alternative proposed solutions have been to start a timer after an unsuccessful DCCH acquisition, and after its expiration recheck the camping suitability of the DCCH. This necessitates the running of a dedicated timer within the mobile station's operating system and applies a fixed time for rechecking that may not be optimal for all service providers. Particularly, the service providers would have no control over how often the mobile station leaves the serving ACC to find service on a DCCH. Still another proposed solution does not recheck the camping suitability of the DCCH again as long as the mobile station remains on the same ACC. However, this could result in the mobile station not acquiring service on the DCCH at some point in the future, even though DCCH service is desired over ACC service.
The present invention is directed to solving one or more of the problems discussed above in a novel and simple manner.